The invention relates generally to Digital Subscriber Line (DSL) systems and particularly to power reduction in DSL line cards.
With the increasing popularity of the Internet, there has been a corresponding increase in the demand for high rate digital transmission over the local subscriber loops of telephone companies. A loop is a twisted-pair copper telephone line coupling a user or subscriber telephone to a central office (CO).
Conventional data communication equipment uses the voice band of the subscriber loop. Such equipment includes voice band modems, which operate at up to 56 kbps using compression techniques. Conventional voice band equipment is limited by the maximum data rate of the existing switching networks and PCM (Pulse Code Modulation) data highways.
Utilizing the frequency bandwidth of the loop outside the voiceband has enabled other high-speed systems to evolve. However because signals degrade with distance traveled, media quality and and transmission characteristics depending on the network, there are limitations and challenges for designers of high-speed systems.
Current high-speed digital transmission systems of the above type include asymmetric, symmetric, high-rate, and very high-rate digital subscriber loops, conventionally known as ADSL, SDSL, HDSL and VDSL respectively. These and similar protocols are known as xDSL protocols.
Of these ADSL is intended to co-exist with traditional voice services by using different frequency spectra on the loop. In today""s ADSL systems, the plain old telephone services (POTS) use the frequency spectrum between 0 and 4 kHz and the ADSL uses the frequency spectrum between 30 kHz and 1.1 MHz for data over the telephone line. ADSL also partitions its frequency spectrum with upstream (subscriber to CO) transmission in a lower frequency band, typically 30 kHz to 138 kHz, and with downstream transmission in a higher frequency band, typically 138 kHz to 550 kHz or 1.1 MHz. ADSL uses a discrete multi-tone (DMT) multi-carrier technique that divides the available bandwidth into approximately 4 kHz sub-channels.
Much effort is being expended by various xDSL hardware manufacturers to reduce overall power dissipation of xDSL line cards. Although overall power reduction improvements have been made, significant power improvements in the area of line drivers have not occurred. To make xDSL technology more attractive, the overall power dissipation must be reduced beyond the presently offered solutions. This power dissipation manifests itself in the form of increased operation temperature of the equipment. Current designs to maintain circuits at a reasonable operating temperature include the use of additional fans, air conditioning, heat sinks and space for thermal ventilation. These constraints significantly increase the material, labor and maintenance cost associated with such a system. Furthermore, excessive heat may restrict the density of equipment, thereby increasing the size of the facility hosting the system and/or limiting the number of customers that can be served by a fixed size facility. Thus, reducing the power consumption in xDSL communications systems can be a key aspect of any system design.
A line driver is an amplifier system which consumes a significant amount of power and can dissipate a significant amount of heat. Typically, a line driver includes an amplifier which receives an analog signal from a preceding circuit, such as a digital to analog (DA) converter on the xDSL line card and drives this signal through a source resistance and a line transformer onto the twisted pair telephone line forming the current loop between the CO and the subscriber. The xDSL line drivers commonly in use include a source or feed resistance equal to the reference impedance of the loop, usually 100 ohms implemented as a series resistance. Typical line drivers use two amplifiers working differentially.
Various forms of line interface circuits are known, and which are particularly applicable to POTS systems. For example, U.S. Pat. No. 5,258,713 describes an impedance generator for a telephone line interface circuit which uses a sensing circuit coupled to the feed resistors in series with the tip and ring lines. The sensing circuit produces a feedback signal for use by an impedance generator circuit. U.S. Pat. No. 5,661,794 describes a telephone line interface circuit, wherein the loop current and common mode current through the feed resistors are monitored and converted to digital signals for providing programmable control of the operating conditions of the circuit. Other exemplary line interface circuits are described in U.S. Pat. Nos. 4,764,956, 5,052,039 and 5,333,192. Thus it appears that without exception known line interface circuits, for use either with POST or xDSL systems, utilize a series feed resistance to match the impedance on the line, which is relatively wasteful of power.
FIG. 1 is a block diagram of a typical prior art xDSL line card, represented generally by the numeral 10. The card 10 couples a twisted pair wire 11, or subscriber loop (consisting of TIP and RING lines), to an xDSL transceiver 12 via a loop interface 15. The loop interface typically includes a transformer 16 having its primary winding coupled across the TIP and RING lines and its secondary winding coupled to the xDSL transceiver, splitting the loop into a customer premises side and a CO side. The transceiver 12 includes a sense circuit 13 for measuring the voltage across a set of resistors R1 and R2, inserted in series in the TIP and RING lines on the secondary side of the transformer. The resistors R1 and R2 can form part or extremely the termination impedance of the transceiver 12. The output of the sense circuit 13 is typically a voltage that is proportional to the current through the resistors R1 and R2. The output of the sense circuit 13 is adjusted by a programmable or automatic gain stage 14. The output of the gain stage 14 is filtered by an analog filter 16, and converted to the digital domain by an Analog-to-Digital Converter (ADC) 18. The output of the ADC 18 is sent to a Digital Signal Processor (DSP) 20 or xDSL receiver for processing.
The resistors R1 and R2 are connected to the output of respective line drivers, 22a and 22b, and respective TIP and RING lines to the secondary of the transformer 16. The line drivers serve to drive an output from an xDSL transmitter 21 onto the loop. Therefore, for a given voltage to be driven onto the loop 15, the loop drivers 22a and 22b must deliver a voltage that is higher than the loop voltage in order to account for the voltage drop across the resistors R1 and R2. The voltage drop across the resistors R1 and R2 is referred to as the insertion loss. Insertion loss increases the power dissipation during transmission to the loop and degrades the overall efficiency of the transmitter. As a consequence, what is needed is to obviate or mitigate at least the above disadvantage.
In particular, what is needed is a mechanism compatible with an xDSL loop driver circuit such as that described in U.S. patent application Ser. No. 09/590,890 assigned to the assignee of the present invention.
In accordance with this invention, there is provided a current sense circuit for sensing a current in a subscriber loop comprising a transformer having a first and second winding coupled for sensing a differential current in said loop, a sense winding for producing a sense current in response to a magnetic flux induced in the transformer due to the differential current, an amplifier circuit for receiving the sense current, the current being indicative of a signal on the loop and configured such that a transformer has a low insertion loss at a frequency of operation of the loop.
In a specific embodiment, the low insertion loss current sense circuit employs a transformer with three windings to sense the differential loop current and two amplifiers to produce a voltage (V1xe2x88x92V2) which is proportionate to the differential loop current and which is for further signal processing.
In a still further embodiment the sense circuit is part of an integrated POTS/xDSL line card.